Accurate control of the power transmitted by a radio is important for a variety of reasons:                Regulatory bodies, such as the FCC, limit the maximum power of radio transmissions.        Transmitted output power level can have an uncertainty on the order of 10 dB without closed loop control.        Radio link range is a function of transmitted power such that higher power results in a link that can be maintained over a longer distance.        Accurately controlling the transmitted power to the minimum needed to maintain a link minimizes interference with other radios and minimizes power dissipation.        
Most radio transmitters use some form of closed-loop control as shown in FIG. 1 to control output power. The transmitter is represented as a modulator 10, which outputs a modulated radio-frequency signal. The output of modulator 10 typically is amplified in multiple stages at least one of which is a variable-gain RF amplifier (VGA) 11, and another of which is a power amplifier (PA) 12. The output of PA 12 is then transmitted out antenna 13. A directional coupler 14 at the output of the PA 12 produces a power sensing signal that is proportional to the power transmitted by antenna 13. The power sensing signal is detected by the power detector 15 which produces a dc voltage proportional to the power sensing signal at its input. The output of power detector 15 is input to a feedback amplifier 16, which controls the gain of the VGA 11 to minimize the difference between the output of power detector 15 and a reference voltage input (Ref) to the feedback amplifier 16. The circuit shown in FIG. 1 may be part of a half-duplex transceiver in which the transmitter and the receiver alternately cycle such that they are not both on at the same time.
Power detector 15 is typically either a diode detector or a logarithmic amplifier. Diode detectors are inexpensive, but they have limited dynamic range (˜20 dB) and suffer from temperature dependence. Logarithmic amplifiers can be very accurate, but their frequency range is limited. Present state-of-the-art logarithmic power detectors have a maximum frequency on the order of 2.5 GHz. That frequency can be increased, but only at the expense of increased power dissipation.
In addition, the analog closed-loop system shown in FIG. 1 is more complicated if the rf output from modulator 10 includes a varying modulation envelope, since the loop will attempt to regulate away the varying envelope. This effect can be addressed by using a partly digital loop which digitizes the output of power detector 15.